The present invention pertains to removal of moisture or water vapor from gaseous hydrogen chloride (HCl).
Gaseous hydrogen chloride is one of the key specialty gases used in the manufacture of semi-conductor devices. HCl is used to grow silicon crystals (epitaxy) and is also used to etch silicon wafers in a dry etching process. For both of these applications high purity HCl is required. High purity HCl is generally taken to mean HCl free of water, or containing less than 0.5 ppm of water.
HCl containing water is highly corrosive and will present problems to the gas delivery system as well as adversely affecting production yields during the manufacture of semi-conductor devices.
For example water contained in hydrogen chloride gas will necessitate frequent replacement of piping, manifolds, valves and the like which are used to deliver the hydrogen chloride to the point of use in the semi-conductor manufacturing process. In cleaning susceptors which are the support structure on which wafers are processed, water contained in gaseous hydrogen chloride will result in a formation of a new oxide on the susceptor thus, preventing the cleaning function of the hydrogen chloride. In etching applications, water contained in the hydrogen chloride becomes a source of undesirable contamination in a semi-conductor manufacturing environment, which may render chip products made in this environment less than efficient or totally useless for their intended purpose.
U.S. Pat. Nos. 4,853,148 and 4,925,646 disclose a number of adsorptive compositions and methods for removing water from HCl. Among the materials and processes suggested is adsorption using magnesium chloride (MgCl.sub.2) supported on an alumina (Al.sub.2 O.sub.3) substrate. However, it has been found that the use of MgCl.sub.2 on Al.sub.2 O.sub.3 for moisture removal from hydrogen chloride results in hydrogen chloride slowly reacting with the Al.sub.2 O.sub.3 support to produce a volatile product, AlCl.sub.3. The AlCl.sub.3 will solidify at cool spots in the downstream process equipment resulting in deposits on the process equipment, such as regulators, filters and valves. This in turn causes system maintenance problems and eventually contamination of the HCl product.
Japanese Patent Application H5-13695 discloses removal of water from gaseous hydrogen chloride using a synthetic mordenite. However, it was found that the synthetic mordenite is not stable in gaseous hydrogen chloride.
Russian Patent 1726368 discloses removing water and oxygen from hydrogen chloride gas using iron chloride (FeCl.sub.2) supported on a carbon substrate.
Prior art workers have also used silica gel to remove water from gaseous hydrogen chloride. However, although silica gel and FeCl.sub.2 on carbon are stable in hydrogen chloride their water adsorption capacities are relatively low so that these materials must be changed more frequently when used in a HCl purification system.
Vacuum impregnation techniques are known in the prior art for some other applications, such as: 1) treating wood to change its color, U.S. Pat. No. 4,959,076 and U.S. Pat. No. 4,840,637, 2) making a carbon-metal phosphate composite, U.S. Pat. No. 4,707,299, and 3) coloring granite, U.S. Pat. No. 4,695,487.
For making an adsorbent, however, a different vacuum impregnation technique was applied in the prior art, which includes: 1) depositing iron (III) chloride on activated carbon for adsorption of hydrogen sulphide, DD Patent 236,912 Al and 2) impregnating an aqueous solution of an alkali salt of N-methylalanine into a porous support for adsorption of carbon dioxide, JP Patent 61-101244 A. In both of these two patents, the term "vacuum impregnation" used the technique of immersing a porous carrier in an aqueous solution under a reduced pressure.
Additional prior art of general interest include; U.S. Pat. No. 2,196,246; U.S. Pat. No. 5,202,106; U.S. Pat. No. 5,539,998; U.S. Pat. No. 5,766,565; DD Patent No. 215,064 and GB Pat. No. 2,188,043.
The prior art has suggested various materials and techniques for impregnating adsorbents and water sorptive materials on supports. In some instances, vacuum has been used for adsorbents for other than water sorption. The vacuum is used for degassing during impregnation. However, the problem still exists to produce an effective water sorptive material to remove water from potentially corrosive gases such as HCl where the water sorptive material is stable in the presence of wet HCl and sufficient loading of the reagent forming the water sorptive material is achieved on a stable support. The present invention unexpectedly achieves these goals and overcomes this long term problem in the industry by the use of effective water sorptive material reagent on a stable support using a vacuum impregnation technique as will be described below.